Technology for Polarization Measurement

# Hinds Instruments Glossary

Absorption: In a light beam, reduction of intensity or energy in the beam due to conversion of light energy to another form, e.g. heat.

Acceptance angle: As used here, this is the angle that  light can enter the PEM, as measured from normal, and  still be modulated with 90% uniformity.

Adsorption: In thin films, the process by which an ultra-thin layer of one substance forms on the surface of another substance.

Astronomical polarimetry: Astronomy, astrophysics, or cosmology studies that are carried out using a Stokes polarimeter.

Angle of Incidence: The angle between the beam of incident light and the surface normal which is perpendicular to the surface. 1

Anisotropy / anisotropic: Referring to a non-uniform spatial distribution of physical or polarimetric properties. 2

Bessel function: A mathematical function that is important for solving problems of wave propagation. They are used to predict optical signals for optical systems that include PEMs.

Birefringence: A difference in the birefringent index of a material that depends on the polarization of light passing through it. The effect includes a change of phase between the s and p polarizations and a change of direction between these two polarization directions.

Chiral media/ chiral molecules: Chiral molecules are those that are not superimposable on their mirror image, e.g. the molecules have “left-handed” and “right-handed” isomers.

“Chopping”: Modulation of the intensity of a light beam that occurs when the PEM is placed between two crossed polarizers.3

Degree of polarization (DOP): A ratio given by the power in a polarized part of an electromagnetic wave to the total power of the electromagnetic wave.4

Demodulation: As opposed to modulation of a signal, this involves the reversal of a modulation effect by removing the modulation from the analog signal.1

Diattenuation: A property of a transmitting material wherein the optical transmittance depends on the incident polarization state of the incoming light.5

Dichroism: A differential absorption of light between two different polarization directions as a light wave propagates through a material.

-circular: The differential absorption (A = AL – AR) between left and right circularly polarized light in the UV-Visible region. It is a measurement of the optical activity of chiral molecules.6

-linear: The differential absorption between two orthogonal, linearly polarized states (A = AX – AY). LD is a measurement of the sample’s bulk property that is a result of the regular orientation of the molecules in the sample.6

-vibrational circular: The differential absorption between left and right circularly polarized light (A = AL – AR) associated with vibrational molecular transitions. It is a measurement of the optical activity for chiral molecules.6

-vibrational linear: The differential absorption between two orthogonal, linearly polarized states (A = A|| – A) associated with vibrational molecular transitions. It is a measurement of the regularity of molecular orientations in a bulk sample.6

-magnetic circular: The differential absorption of left and right circularly polarized light induced by a longitudinal magnetic field between different electronic states.6

Differential absorption: The difference in absorption in a material between two orthogonal polarization states of light.1

“Double modulation”: Literally to modulate twice. This most commonly refers to modulation once by a spectroscopic system and then once again by a photoelastic modulator. This would of course require that the output signal be demodulated twice.

Electric field: An effect produced by an electric charge (or a time-varying magnetic field) that exerts a force on another electric charge in the field.

Ellipsometry: A method for determining the properties of a material from the polarization characteristics of elliptically polarized incident light reflected from its surface or the transmitted material.7

Excitation: Referring mainly to being in an excited state where a sample is in a state that has more energy that in its ground state (the state with the minimal amount of energy).1

Faraday rotation: An interaction between light and a magnetic field in a material sample. The rotation of the plane of polarization is proportional to the intensity of the component of the magnetic field in the direction of the beam of light.1

Fast axis: The axis of a birefringent material that aligns with the faster moving component of the polarized light as it passes through the sample.2

Flow cell – cuvette: A type of laboratory glassware that is used for optical measurements and is made from an optical grade transparent material (e.g. plastic, glass, or quartz).1

Fluorescence: The emission of electromagnetic radiation stimulated by the absorption of incident radiation.8

Fused Silica: A type of non-crystalline glass or silica this is fused into a single structure providing optical and thermal properties that are superior to other types of glass.1

I Values: See Stokes Parameters.

Index of Refraction (refractive index): For an optical material, the ratio of the speed of light in a vacuum to the speed of light in the material.

IRRAS (Infrared Reflectance/Absorption Spectroscopy): The differential IR absorption between s- and p-linearly polarized light for the molecules adsorbed on a metal surface. The effect is observed in reflection from the surface and is wavelength-dependent.

Interferometry: The study of waves when two waves are combined to interfere with each other. In some applications this is used to combine light from two or more sources to obtain measurements with higher resolution than could be obtained from either source individually.1

Isotropic (optical): Having the same optical properties in all directions.1

Kerr effect / Kerr rotation: The non-linear magneto-optic effect that explains the changes in polarization of light reflecting from a magnetized media.1

Luminescence: Light emission that can be caused by chemical or biochemical changes, electrical energy, subatomic motions, reactions in crystals, or stimulation of an atomic system.1

Magnetic Field: An effect produced by an electric current that exerts a force on a second electric current (or moving electric charge) that exerts a force on the second current. Two parallel currents propagating in the same direction attract, whereas those with opposite directions repel.

Modulation: The process of varying with time a property of a light beam (e.g. polarization state or amplitude).

MOKE: Magneto-Optic Kerr Effect, or the study of the reflection of polarized light by a material sample subjected to a magnetic field.9

Mueller polarimetry: Polarimetry used for measuring the matrix elements in Mueller matrices.

Mueller Matrix: See Stokes vectora 4x4 matrix for calculating the effect of a polarizing element (polarizer, waveplate, etc.) on the polarization state (Stokes Vector) of a light beam.

Normalized Stokes parameters: Stokes parameters (I, Q, U, V) that have been divided by I, where I is the incident irradiance. The Stokes parameters for unpolarized light then become (1,0,0,0).10

Optical activity: The property of rotating the plane of polarization of light as it passes through the material. Optically active materials may also exhibit circular dichroism.

Optical rotation: The rotation of the plane of linearly polarized light as it passes through an “optically active” material.11

Photomultiplier Tube (PMT): A sensitive electro-optic detector designed to amplify the photocurrent produced by light striking a photo-sensitive cathode. The current gain can be controlled by varying the high voltage used to operate the PMT.

Photonic spectrum (Ultraviolet light, Visible light, Infrared light)

-Ultraviolet light: Subdivided into: Near UV (380–200 nm wavelength), Far, vacuum, or deep UV (200–10 nm; abbrev. FUV, VUV, or DUV), and Extreme UV (1–31 nm; abbrev. EUV or XUV).”1

-Visible light: Light to which the human eye responds, typically 400 to 700nm.1

-Infrared light: Subdivided into: Near infrared (750nm-3µm; abbrev. NIR), Mid infrared (3-30µm, abbrev. MIR), and Far infrared (30µm-1000µm, abbrev. FIR)1

Plane of incidence: The plane created by the incident beam and surface normal.

Polarimetry: Measurement of polarization properties of a sample including optical rotation, Stokes Polarimetry, and Mueller Polarimetry.11

Polarization state: A description of the polarimetric properties of an electromagnetic wave.4

Poincaré sphere: A standardized spherical model that represents polarized light and can also be used to determine how different waveplates and retarders will change the polarization of light.1

Polyethylene Terephthalate (P.E.T.): Thermoplastic polymer resin in the polyester family commonly used in food and beverage containers.

Q values: see Stokes Parameters.

Reflection: The change of direction of a wavefront between two dissimilar media. The angle at which a light beam hits a surface is referred to as the angle of incidence (Ti) and is equal to the angle of reflection(Tr).1

Refraction: The change in the direction of a wave due to a change in velocity. This is most frequently seen when a light source passes from one medium to another.1

Residual Birefringence: Birefringence that is intrinsic to a material. It may be caused by crystal defects, manufacturing process, or maybe present naturally due to anisotropies in crystal structure. it may also be called "static birefringence". It is not time dependent, as is the "induced birefringence" in a PEM.

Resonance: The frequency or frequencies at which an object is most easily vibrated.

Retardation: The integrated effect (on the relative phase) of two orthogonal polarized light beams traveling along a path in a birefringent material.

Retarder: See waveplate.

Rheology: The study of the deformation and flow of matter under the influence of applied stress.1

s- and p- polarized light: The parallel and perpendicular components of an electric field, termed "p-like" (for parallel) and "s-like" (for senkrecht, which is perpendicular in German). The p- component lies in the plane of incidence of an electric field, and the s- component lies perpendicular to the plane of incidence .1

Slow axis: The axis of a birefringent material that aligns with the slower moving component of the polarized light passing through a sample.

Spectroscopy: The study of spectra, that is, the dependence of physical quantities on frequency. This is often performed by exciting the electrons of a sample and analyzing the emitted light waves of the excited sample.1

Stokes Parameters: A set of four numerical parameters that describe the polarization of a light source, as can be determined experimentally.

I = total intensity; proportional to the DC signal
Q = linear polarization component at 0° or 90°; the 2f frequency of PEM2
U = linear polarization component at 45°; the 2f frequency of PEM1
V = circular polarization component; the 1f frequency of PEM 1

Stokes polarimeter: An instrument for measuring the Stokes parameters.

Stokes vectors: a 1x4 matrix whose elements are the Stokes parameters – see Mueller Matrix.

Transmission: The fraction of incident light at a specified wavelength that passes through a sample.1

U Values: See Stoke Parameters.

V Values: See Stokes Parameters.

Waveplate, retarder: An optical device used to shift the phase of a light wave between two perpendicular polarization components.1

½ wave waveplate: A lightwave retarder designed to shift the phase of two perpendicular polarization components of a light beam by ½ wave.

¼ wave waveplate: A lightwave retarder designed to shift the phase of two perpendicular polarization components of a light beam by ¼ wave.

Zeeman effect / Zeeman splitting: The splitting of a spectral line into several components in the presence of a magnetic field.1

Zero order waveplate: A lightwave retarder designed to minimize dispersion of a lightwave as it propagates through the waveplate.

3 Light Intensity Modulation Using a PEM, www.hindsinstruments.com

4
www.ccrs.nrcan.gc.ca/glossary

7 PEMs in Ellipsometry Application Note, www.hindsinstruments.com

9 Magneto-Optical Kerr Effect Application Note, www.hindsinstruments.com

10 Optics, Hecht and Zajac, The Stokes Parameters, p.267

11 Polarimetry: Optical Rotation Application Note, www.hindsinstruments.com